The present invention relates to a series of improvements introduced in the cold start system of diesel engines, whereby an optimisation is achieved of said starting in the ambit of unitary cylinder capacities of the order of 200 to 650 cubic centimetres.
The object of the invention is the development of a system for the uniform heating of the intake air flow in direct-injection diesel engines of small unitary cylinder capacity through an electrical resistance, in the form of a continuous and optimally distributed tape in the section of passage of said fluid, to achieve the starting of such engines at temperatures below xe2x88x9210xc2x0 C. and without impairing the performance of the engine in conditions of maximum power operation as a consequence of a high head loss in the heating element.
High-speed direct-injection diesel engines employed in motor vehicles and with unitary cylinder capacities of the order of 200 to 650 cubic centimetres, are being used massively in substitution of the divided-chamber or indirect-injection engines, mainly due to the smaller specific consumption of direct-injection engines.
Also, with relatively high supercharging pressures and due to the new concept of fuel injection systems at constant pressure, it is possible to obtain specific powers of the order of 60 kW/l, which makes that type of diesel engine very competitive when compared with petrol engines.
However, one of the main problems with diesel engines is associated with cold starting, especially in severe climates where ambient temperatures usually attain very low levels, so that to overcome said drawback or problem, high-speed direct-injection diesel engines use glow plugs (electrical resistances) located, through the head, inside the combustion chamber of the engine, to provoke the self-ignition of the fuel that is present on the surface of said spark plug which in turn initiates the engine combustion process. However, although such a solution is accepted practically universally, it in turn presents a series of drawbacks, problems and difficulties due to three fundamental concepts that can be summarized as follows:
1.xe2x80x94Reduced reliability, through being an element subjected to high mechanical and thermal stress.
2.xe2x80x94Difficulty in location, due to the use in a widespread way of the concept of four valves per cylinder in new-generation diesel engines, so that this solution leaves scant margin for manoeuvring in locating the preheating spark plugs, bearing in mind that the location should assure their interaction with the fuel jet, allow accessibility from the exterior and guarantee an adequate thermodynamic design of the head, a problem that can be augmented by the demand for greater specific powers while simultaneously reducing emission levels.
3.xe2x80x94Influence on the combustion process, since once its function in cold starting has been carried out, the preheating plug is an obstacle located inside the combustion chamber that hinders the process of mixing the air with the fuel, disturbing the range of air speeds and inevitably worsening engine performance, like a poor combustion process for example. Also, in diesel engines of the type described the size of the chamber is reduced and high injection pressures are employed, which leads to a greater number of injector nozzles, to assure a good fuel atomisation, which in turn leads to an improvement in the mixing process and therefore in the combustion. However, the presence of the preheating plug inside the chamber produces an unavoidable interaction with the injection jets that finally imposes a limit to all the improvements introduced in the process, as a consequence of an incomplete combustion process.
To overcome the drawbacks of cold starting in diesel engines it is possible to employ the concept of heating the intaken air by means of a heat source that can be a flame or an electrical resistance as is described in the U.S. Pat. No. 4,512,322. However, these solutions, as they have been conceived, are only possible to apply to the case of engines with unitary cylinder capacities above 800 cubic centimetres, since these engines are more adiabatic and therefore have smaller heat losses. Engines with smaller unitary cylinder capacities require more optimised systems, both from the point of view of heat energy transfer and of head losses. For this reason, it is essential in these engines to perform a uniform heating of the intake air along the entire section of fluid passage, an aspect which is inconceivable with the solution proposed in the patent mentioned, since due to the arrangement of the heating element, the heating would only be achieved in a very small portion of air concentrated in the central area, there still remaining around this an important quantity of air at a very low temperature, which means a very low thermal efficiency in achieving appropriate air temperatures for cold starting in motor vehicle engines. Moreover, the direct drawback associated with a heater distributed over the orifice section of the fluid passage relates to the loss of pressure produced, which in small engines constitutes a critical aspect regarding their performance at high running speeds. This demands a hydrodynamic design of the heater that is far removed from that proposed in the U.S. Pat. No. 4,685,437, a solution conceived with the goal of achieving more uniform air temperatures than with the above-commented solution (U.S. Pat. No. 4,512,322) but with unacceptable values of head loss in small engines under operating conditions different to cold starting.
In the Spanish Patent of Invention 9900616 a diesel engine cold starting system is foreseen also based on performing a preheating of the orifice intake air load, said preheating of the air being carried out before the latter reaches the combustion chamber of the cylinders of the engine. The electrical resistance or resistances are arranged before the intake manifold and/or preceding the intake valves of the engine, so that when the heating energy losses through the walls of the manifold are low, either because the corresponding intake manifold is of a material with low thermal conductivity or for other reasons, generally a single electrical resistance (7) is fitted for the preheating, which is located at the inlet of said manifold, whilst if the losses through the walls are high, for example because the manifold is of a material with high thermal conductivity, a series of electrical resistances are fitted immediately before the inlet of each of the cylinders of the engine.
The improvements object of the present certificate of addition, applicable to the Patent of Invention 9900616 mentioned in the last paragraph of the previous section, supposes a higher functional efficiency of said system, by achieving less resistance to the passage of the air, which implies a smaller head loss, and a greater contact surface between the heating element and the air, which improves the heat transfer.
For this reason and in a more specific manner the improvements of the invention consist essentially in the use as heating medium of an electrical resistance in tape form, optimally distributed over the section of fluid passage and in the direction of the flow, whereby a uniform temperature of the air charge is achieved that the cylinder breathes adequate for starting the engine at low temperatures, as well as perfectly acceptable levels of head loss for the engine under other operating conditions.
It has also been foreseen that the support for fastening the tape resistance to the engine incorporates in diametrically opposed areas of the external mouth of the orifice for the passage of air to the engine, individual groups of recesses of approximately semicircular configuration, with a nipple inside, defining grooves having an arched trajectory in which are mounted and fastened the inflections of the laminar resistance, with the interposition of respective ceramic insulators, there also being two special slots, one blank and lacking an insulator for fastening the corresponding end of the resistance to the support itself, as a connection to ground, through a setscrew, with a rivet or the like, and another open to the exterior, accompanied by a ceramic insulator and provided to house the other end of the resistance, that which is connected to the electricity supply cable.
The special arrangement and configuration of the electrical resistance, means this offers its profile to the air passage, so that the obstructing surface of this resistance is minimum and consequently minimum also the resistance to the passage of air, which implies a remarkable decrease in the head loss. At the same time a greater contact surface is achieved with the air, since for a same section in the resistance, the external surface is considerably greater in the case of a laminate of reduced thickness than in the case of the classic resistances of circular section, which signifies a greater transfer of heat.
The form of the housings for the resistance guarantees the reliability of the heater, since its correct fastening is assured and the resistance suffering modifications in its original geometry produced by thermal dilation is avoided. These thermal dilations of the resistance are absorbed by the arched lodging channels present in the insulating supports that in turn isolate the electrical resistance from the body of the heater.
The employment of an electrical resistance in tape form provides the advantage that the heater is more robust with respect to the mechanical vibrations to which it can be subjected, which can become very intense depending on the operating conditions of the engines, whereby complete protection is also assured of the engine cylinders that could be damaged in the event of detachment of the resistance or part thereof due to breakage.
The electrical resistances in tape form have better mechanical properties, which makes it possible to obtain a system with relatively low thermal inertia due to the geometry and the total mass thereof, which in turn offers a better control of their thermal state even under non-stationary conditions of flow, as for example happens with the intake air flow in the alternative internal combustion engines.
The noticeably coiled arrangement of the heating resistance or element determines a uniform distribution of the heat in the air intake section, whereby uniform heating thereof is achieved.
Apart from this the resistance can be housed in a body external to the intake manifold, specifically in the aforementioned support, or it can be housed in the intake gasket itself, specifically in the case of having to locate a heater before each intake pipe, it being equally possible for the resistances to be housed in the intake manifold itself, either in the inlet, when there is a single heater, and/or in the outlet when there is a heater for each cylinder.